Abstract

Recently, we have shown that a polycrystalline Ni–Mn–Ga magnetic shape-memory alloy, when containing two populations of pore sizes, shows very high magnetic-field-induced strain of up to 8.7%. Here, this double-porosity sample is imaged by x-ray microtomography, showing a homogenous distribution of both pore populations. The orientation of six large grains—four with 10M and two with 14M structure—is identified with neutron diffraction.In situ magnetomechanical experiments with a rotating magnetic field demonstrate that strain incompatibilities between misoriented grains are effectively screened by the pores which also stop the propagation of microcracks. During uniaxial compression performed with an orthogonal magnetic bias field, a strain as high as 1% is recovered on unloading by twinning, which is much larger than the elastic value of measured without field. At the same time, repeated loading and unloading results in a reduction in the yield stress, which is a training effect similar to that in single crystals.

Received 20 October 2010Accepted 01 November 2010Published online 30 December 2010

Acknowledgments:

The authors thank Phil Boysen (Boise State University) for assistance with experimental design and machining, and Adrian Rotherbühler (Boise State University) for the setup of the optical attachment of the magnetomechanical experiment. M.C., C.W., P.M., X.Z., and D.C.D. acknowledge financial support of the National Science Foundation, Division of Materials Research Grant Nos. DMR-0804984 (BSU) and DMR-0805064 (NU). M.C. acknowledges financial support of the German Research Foundation (DFG) through the priority program SPP 1239 (Grant No. Schn 1106). P.M. is thankful to ETH Zürich, Switzerland, for donating magnetomechanical testing devices.